From the viewpoint of global warming prevention and resource saving, an idling stop system (which is also referred to as an economy running system or an engine automatic stop-start system, and hereinafter may be referred to as an eco run system) has been in practical use. The idling stop system automatically stops the engine when the vehicle stops at a red light at an intersection or the like, and restarts the engine when the driver performs an operation to restart the vehicle (for example, presses an accelerator pedal, stops pressing a brake pedal, or moves a shift lever to a forward driving position). When the engine is to be restarted, the system uses power from a secondary battery mounted on the vehicle to rotate a crankshaft by means of an electric motor such as a motor generator or a starter motor to restart the engine.
In such an eco run system, control must be effected to detect various sensor signals and switch signals, stop the engine when a predetermined condition for starting eco run is satisfied, and then restart the engine.
On the other hand, the engine mounted on such an eco run vehicle is often provided with a VVT. There is an example of the VVT that attempts to improve the torque at low-to-middle speed, the fuel efficiency and the emission performance by performing a continuous variable control of the open/close timing of an intake valve and/or exhaust valve by means of an electronically controlled hydraulic drive. In addition to this timing control, there is another example that variably controls the lift amount of the intake valve and/or exhaust valve in order to improve the intake/exhaust efficiency. The VVT is to control the intake/exhaust valve which is the base component of the engine, and requires abnormality diagnosis in case of a failure.
Japanese Patent Laying-Open No. 2004-293520 discloses an abnormality diagnosis device that operates to enlarge the region in which abnormality diagnosis is carried out for a valve timing control system, and that quickly and reliably detects an abnormality. The abnormality diagnosis device functions to diagnose an abnormality of the valve timing control system adjusting the rotation phase between a crankshaft and a camshaft of the engine, and includes means for detecting a change in engine speed associated with the change in the driving conditions to calculate a diagnosis value based on the change in the engine speed, and means for comparing the diagnosis value with a predetermined criterion threshold value to determine that an abnormality occurs when the diagnosis value exceeds the criterion threshold value.
According to the abnormality diagnosis device of the valve timing control system, when the amount of change in the engine speed (or an integration value of the amount of change in the engine speed) corresponding to a change in the operating conditions exceeds the criterion threshold value, the valve timing control system is diagnosed as abnormal. Thus, irrespective of the operating region, a decrease in the response can be quickly and reliably detected, which is caused by a sliding failure or adhesion of a sliding part of the variable valve timing mechanism. Furthermore, not only in the case of adhesion or the state close to adhesion of the sliding part, but also in the case where the hydraulic pressure attaining a level higher than that defined causes a delay in the response of the variable valve timing mechanism to lead to a decrease in the response of the actual advance angle relative to the target advance angle, abnormality can be accurately detected to improve diagnostic accuracy.
However, when stopping the engine, if the engine is stopped in the compression stroke, pistons may be pushed back by compressed air. This is more likely to occur especially in the eco run vehicle because the engine is frequently stopped and restarted in the eco run vehicle. If the pistons are pushed back by compressed air as described above, it is assumed that an abnormality occurs in the VVT when an advance angle command is output from an Electronic Control Unit (ECU) to a VVT controller. This abnormality includes a decrease in the response caused by a sliding failure of each sliding part of the VVT which results from accumulation and deposition and the like of foreign matter due to impurities in the oil and deterioration thereof, adhesion caused by a foreign matter entering the sliding parts, and the like. If the VVT is in an abnormal state due to adhesion and the like when the advance angle command is output, there may be no displacement of the cam position to the advance angle by the VVT. In this case, the VVT should be determined to be abnormal. However, if the engine is stopped in the compression stroke, the pistons may be pushed back by compressed air to thereby slightly push the crankshaft in the reverse direction. On the other hand, since the camshaft is not rotated in the reverse direction, detection is made by a cam position sensor as if there is a relative VVT advance angle amount. This reverse movement of the crankshaft will cause detection as if the cam position advances, such that occurrence of abnormality cannot be detected. Therefore, the engine will be controlled assuming that there is no abnormality in the VVT when the engine is restarted.